The present invention relates to a squirrel-cage induction motor, or more in particular to a squirrel-cage induction motor suitable for a motor driving a pump or a fan and the like.
Generally, induction motors are roughly classified according to the construction of a rotor into a wound-rotor induction motor and a squirrel-cage induction motor.
The squirrel-cage induction motor comprises a rotor so constructed that a bar conductor of considerable size is embedded in a slot of a rotor core and the end of the bar is provided with an end ring. With this construction, the squirrel-cage induction motor is mechanically rugged and high in weatherability, and therefore finds wide applications in various fields. Nevertheless, the squirrel-cage induction motor, which is driven at a predetermined speed based on a commercial frequency, is not suitable for the applications requiring rotations at variable speeds. If a variable speed control is desired by any means, a special device, that is, a frequency converter is used for changing the source frequency as disclosed in Japanese Patent Laid-Open No. 94583/80. The frequency converter is high in cost, and therefore the use thereof increases the production cost of the induction motor to the practical disadvantage. Further, the frequency converter has a limited capacity, thereby tending to limit the capacity of the induction motor.
In the case of the wound-rotor induction motor, on the other hand, the speed control is possible without a frequency converter, and due to the additional advantage of a large starting torque, finds wide applications with pumps or fans and the like. Another advantage of the wound-rotor induction motor is that it can be operated while dampening the starting current unlike in the squirrel-cage induction motor. Specifically, a current 3 to 5 times larger than the rated current flows in the squirrel-cage induction motor at the time of starting, posing the problems of the increase in winding temperature and the difficult smooth increase of speed, while the wound-rotor induction motor can be started smoothly while dampening the starting current by controlling the secondary resistance.
In spite of the many advantages of the wound-rotor induction motor from the viewpoint of operation as mentioned above, it has a number of structural shortcomings. Specifically, in view of the fact that the rotor is wound with the secondary coil, the particular coil or especially the coil ends are subjected to a large outward component of force attributable to the centrifugal force of rotation, thereby weakening the mechanical construction of the motor. Further, the rotor is provided with a collector including a brush and a slip ring for control of the secondary coil, so that a spark is liable to occur from the collector due to the sliding between the brush and slip ring, thus making the motor unusable in an unfavorable environment. Furthermore, the collector device requires great labor and time of maintenance and inspection.
In view of these facts, an ideal induction motor would be the one having the operating advantages of the wound-rotor induction motor and the rugged construction of the squirrel-cage induction motor. In other words, an induction motor having a squirrel-cage rotor and the same functions as the wound-rotor induction motor would be most desirable.